Turbochargers are provided on an engine to deliver air to the engine intake at a greater density than would be possible in a normal aspirated configuration. This allows more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight.
Generally, turbochargers use the exhaust flow from the engine exhaust manifold, which exhaust flow enters the turbine housing at a turbine inlet, to thereby drive a turbine wheel, which is located in the turbine housing. The turbine wheel provides rotational power to drive the compressor wheel of the turbocharger. This compressed air is then provided to the engine intake as referenced above.
Referring in more detail to a representative turbocharger 10 shown in the cross-sectional view of FIG. 1, the turbocharger 10 includes a turbine housing or casing 12 having a volute 14 extending circumferentially therein, a compressor housing 16 and a compressor volute 17, and a turbine wheel 18 and a compressor wheel 19 that are rotatably connected together by a shaft 21. The shaft 21 is supported by a bearing system 22 which is supported within a central bearing housing 23 disposed between the turbine housing 12 and the compressor housing 16. The bearing housing 23 defines a bearing chamber 24 which extends axially between the compressor housing 16 and turbine housing 12 to allow the shaft 21 to extend axially therebetween so that rotation of the turbine wheel 18 drives rotation of the compressor wheel 19 during operation of the turbocharger 10.
In this illustrated turbocharger, the bearing system 22 typically includes a journal bearing positioned within the bearing chamber 24 to provide radial support to the rotating shaft 21. For reference purposes, the Z direction extends axially along the central axis of shaft 21, while the X and Y directions extend radially outwardly therefrom. An end view of the shaft 21 and turbocharger 10 is viewed along the Z direction while the side view is taken in the X direction. These directions are for reference and viewing purposes and are not intended to be limiting.
Generally, the journal bearing 25 is able to float or move radially to a small extent during shaft rotation. The floating ring bearings 25 function to radially support the shaft 21 and prevent radial contact between the shaft 21 and bearing housing 23 in response to radial loads on the shaft 21.
To dampen the radial and axial movements caused by shaft rotation, a fluid such as oil is supplied to the bearing chamber 24 which oil is able to flow around the journal bearing 25 and shaft 21. The oil is supplied to the bearing chamber 24 through an inlet passage 26 which feeds two oil feed ports 27 that supply oil to the journal bearing 25 in a dual-feed configuration for the bearing housing. The bearing housing may also supply oil in a single-feed configuration. The oil thereby surrounds the outside and inside circumferential surfaces of the journal bearing 25. During shaft rotation, an inner fluid film is formed on the inside bearing surfaces which define a journal bearing supporting the shaft 21 radially. The outside bearing surfaces also have an outer fluid film formed thereabout which provides radial support to the journal bearing 25 relative to an inside surface 28 of the bearing chamber 24. The outer and inner films form between the outer and inner bearing surfaces and the respective chamber surface 28 and outer shaft surface 21A. If desired a thrust bearing may also be provided.
In one known journal bearing, a spiral groove may be provided on the inner bearing surface at one end which serves to pump the oil toward the one end of the journal bearing. The spiral groove opens inwardly in close association with the outer shaft surface and extends lengthwise to the one end which generates an axially directed pumping effect during shaft rotation. The spirals are typically oriented such that when oil is pumped, it is pumped axially along the shaft toward the more critical turbine side of the turbocharger. This can generate an oil starvation issue during cold start due to the pumping effect being directed toward the turbine side and away from the compressor side.
Further, these known journal bearings are usable for one direction of shaft rotation which corresponds with the orientation of the spiral. In other words, the spiral is oriented to pump axially toward the turbine when the shaft rotates in one direction but will function to generate the pumping effect toward the compressor when the shaft rotates in the opposite direction.
As such, disadvantages can still exist with this known bearing configuration.